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This week's list of the most horrendous health-care stocks is brought to you by the letter "S." Just like on Sesame Street, all of the names we will discuss start with the same letter. Here are the three swiftest-sinking stocks in the sector for the week.

Sequenom attributed lower-than-expected top-line and bottom-line numbers to billing code changes made by the Centers for Medicare and Medicaid Services and insurance companies. As a result of these changes, payments to the company were delayed. To make matters worse, Sequenom switched from an outsourced billing provider to an in-house system during the quarter. The external provider didn't collect nearly as much in payments due as was expected.

Even though Sequenom expects the payment delay issues to be largely resolved in the second half of the year, analysts didn't wait long to begin downgrading the stock. Those downgrades came partly because of comments made the company made about increased competitive pressure for its MaterniT21 diagnostic tests.

Accelerated anxietySarepta Therapeutics (NASDAQ: SRPT) announced what initially seemed like good news on Wednesday. As investors digested that news, though, it didn't sound so great. Sarepta's stock fell almost 17% for the week.

The biotech stated that it plans to move ahead with submitting a New Drug Application, or NDA, for Duchenne muscular dystrophy drug eteplirsen based on phase 2 clinical data. However, the Food and Drug Administration wouldn't commit to an accelerated approval pathway for the drug.

Uncertainties related to whether Sarepta will be able to gain approval for eteplirsen led several analysts to downgrade the stock. While the FDA did say that it is "open to considering an NDA" based on clinical data provided by the company, those words don't give anyone a warm-and-fuzzy feeling.

Bracing for bad news?Unlike Sequenom and Sarepta, Synta Pharmaceuticals (NASDAQ: SNTA) didn't announce earnings or a major decision about a drug. But its shares still sank by nearly 12% this week. What gives?

This appears to be a case of some investors bracing for bad news. Synta announces its second-quarter results next week on Aug. 1. With no products on the market and no significant revenue to speak of, it's not as if anyone expects great financial numbers for the quarter. However, like other development-stage biotechs, Synta focuses primarily on clinical updates during its earnings conference call. My guess is that there could be jitters about what the company says.

Just last week, the company announced publication of results from clinical studies that showed experimental drug ganetespib could offer a novel way of preventing cancerous tumors from growing new blood vessels. Synta also released positive findings in June from an interim survival analysis in an ongoing phase 2/3 trial of the drug as a second-line treatment of non-small cell lung cancer. Perhaps some investors are now apprehensive about seeing the company follow up with less positive findings with its update next week.

"S" marks the spotWhich of this week's health-care stinkers could lead to hidden treasure for intrepid investors? I'd cross Sequenom off the list. The stock already ranks as one of the worst-performing biotechs of the past decade. Unfortunately, I don't see it improving anytime in the near future.

Sarepta and Synta, though, could easily be winners over the long run. While I'm not sure if the FDA will approve eteplirsen based only on phase 2 data, I think the drug will ultimately gain approval. It's only a matter of time, in my view, before shares rebound significantly. I like Synta's prospects with ganetespib, also.

While there is plenty of risk with both stocks, I don't think either will remain horrendous forever. Sometimes on biotech treasure maps, "S" marks the spot.

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Keith began writing for the Fool in 2012 and focuses primarily on healthcare investing topics. His background includes serving in management and consulting for the healthcare technology, health insurance, medical device, and pharmacy benefits management industries.